Traditional preparation methods of an amino acid-producing strain include increasing the amount of a gene expressed on the biosynthetic pathway of an objective amino acid, releasing feedback inhibition and transcription inhibition by an object project, and increasing the supply of precursor by intensifying a gene on the central metabolic pathway. In other words, the traditional breeding method was focused mainly on the cultivation of a strain whose synthesis is not easily inhibited by an excessive production of objective amino acid in a cell.
In recent years, however, active studies have been performed on various amino acid importers/exporters for use in the preparation of an amino acid-producing strain. The studies are aimed to protect many enzymes on the biosynthetic pathway from the feedback inhibition and the transcription inhibition by an object product. This is made it possible by reducing the concentration of a particular amino acid in a cell through the importer defect or the intensification of the exporter of that particular amino acid. For instance, the report on lysine exporter (lysE) of Corynebacterium glutamicum (Microbiology, 147:1765, 2001), and the report on the threonine production improvement by expressing threonine exporter (thrE) of Corynebacterium glutamicum from E. coli (Appl. Microbiol. Biotechnol., 59:205, 2002) are some of the examples of the intensification of the exporter to increase the yield of a specific amino acid. As mentioned the above, the importer of a specific amino acid can also be defected to increase the yield of the amino acid. For example, the yield of tryptophan was increased by a mutant strain defective in the importer of an aromatic amino acid of Corynebacterium glutamicum (Biosci, Botech. Biochem., 59:1600, 1995), and a strain defective in the threonine importer was prepared from E. coli to increase the yield of threonine (Biosci. Botech. Biochem., 61:1877, 1997).
In this light, the present inventors have tried to prepare a threonine-producing strain from Corynebacterium glutamicum based on the discovery that the concentration of intracellular threonine was reduced and the feedback inhibition and the transcription inhibition by threonine of a threonine biosynthetic gene could be prevented by blocking transfection of threonine of high concentration into a cell, which was made it possible by defecting the threonine import pathway. That is, a threonine importer was identified and defected to produce the threonine-producing strain from Corynebacterium glutamicum. 